Structures and Buildings


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Structures and Buildings

  3. 3. STRUCTURE In engineering and architecture, a structure is a body or assemblage of bodies in space to form a system capable of supporting loads. Physical structures include man-made and natural arrangements. Buildings, aircraft, soap films, skeletons, anthills, beaver dams and salt domes are all examples of physical structures. The effects of loads on physical structures are determined through structural analysis. Structural engineering refers to engineering of physical structures. Built structures are a subset of physical structures resulting from construction. These are divided into buildings and non-building structures, and make up the infrastructure of a human society. Built structures are composed of structural elements such as columns, beams and trusses. Built structures are broadly divided by their varying design approaches and standards, into categories including Building structures, Architectural structures, Civil engineering structures and Mechanical structures.
  4. 4. FORCE AND LOAD The influence of the environment on structures takes the form, principally, of Loads and Forces. Here the word 'environment' is taken to mean anything in contact with the structure (e.g. vehicles, furniture, people etc.), including the structure itself. Such primary environmental influences as wind, temperature, and earthquake affect the structure by exerting forces on it.
  5. 5. FORCE AND LOAD FORCE : Influence on a body, causing (or attempting to cause) the Movement of the body or part of it, or causing a change in its movement, if it is already in motion. This is the common definition of force encountered in the literature. It is interesting to note that even though force is one of the most fundamental concepts in physics, its definition is indirect, relying on its effect. This is an indication of the complexity of this concept and the difficulty in visualising it. The definition of force through the concept of motion, a concept which is easy to grasp intuitively, enables easy visualisation of forces, and brings forth the extremely important relationship between force and motion. This, in fact, is the source of the force-shape relationship which is the focal point of this work. A force is a Vector. A vector is a parameter (a physical quantity) characterised by a magnitude, (or 'intensity') and a direction. Relying on the correlation between force and motion, it is convenient to visualise a vector in terms of motion: when an object moves from point A to point B, the magnitude of the distance travelled is not enough to define the position of point B, relative to A. We need to know the direction as well. Distance, like force, is a vector. A vector is described graphically as an arrow, pointing in the direction of the vector and having a length representative of the magnitude.
  6. 6. UNITS OF MEASUREMENT Force, like distance, is one of the fundamental physical entities, measured in one of the basic units. The basic force unit is the Newton (denoted N) and its multiples -kilo-Newton (kN, one thousand Newtons) and Mega-Newton (MN, one million Newtons). As a rule, the international system of units is used throughout this text, with some exceptions. This system employs the Newton (N) and its derivatives for force units, and the metre (m) or millimetre (mm) as length units. Centimetre (cm) is also used occasionally.
  7. 7. LOAD A force applied to a structure by the environment or by any object (including the structure itself or other structures). Alternative definition: any External force applied to the structure, other than a Reaction force.
  8. 8. TYPES OF LOAD The structures in question are buildings, bridges, monuments, signposts etc. There are two major types of loads: Gravity loads, which are usually vertical, and Environmental loads, which are often horizontal (e.g. earthquake) but can generally take any direction. Although all loads were defined as arising from the influence of the environment, the term Environmental load refers to a subclass of loads defined below. TYPES OF GRAVITY LOAD: Gravity loads are the effect of the weight of objects on the structure, including the weight of the structure itself (weight is a force). Two kinds are distinguished: Dead load: Load resulting from the self weight (SW) of the structure and of any permanently attached components, such as walls, flooring, permanent partitions etc. Live load: Load arising from the function of the structure, including attached components whose location is not fixed, such as movable partitions. Live loads are a result of the weight of the loading objects (vehicles, furniture, goods, people etc.) and are mostly vertical (snow load is also considered live load). In some cases, however, loads may be applied in non-vertical directions, for instance loads due to braking of vehicles, loads transmitted through pulleys, earth or hydrostatic pressure etc.
  10. 10. ENVIRONMENTAL LOAD Environmental loads are not a direct result of the weight of objects, but of movement in the structure's environment. The most common environmental loads are Wind load and Earthquake load. Wind load is a result of moving air tilting the structure. Earthquake load is a result if the movement of the earth in which the structure is founded. The force-movement relation is reciprocal. In the same way that force causes movement, force can be caused by movement. In the above instances, the movement (of the air or the ground) causes forces on the structure and these forces, in turn, cause movement of the structure and of parts of the structure relative to one another.
  11. 11. OTHER ENVIRONMENTAL INFLUENCES Other environmental influences are movements which may cause Internal forces in certain structures. In other cases they only cause Deformations. These influences include temperature effects -change of temperature or temperature difference over parts of the structure, e.g. between the inside and the outside; Support settlement settlement (sinking) of foundations by differing amounts; and so on. Some other influences affecting dimensions of components of the structure are also considered environmental effects because of the similarity to the influence of temperature and settlement. These include statistical variation in component dimensions ('lack of fit'), and deliberately induced Deformations.
  12. 12. LOAD DISTRIBUTION Load has been described in general terms, as the overall force acting on the structure, causing movement in it. In practice, a load applied to a structure is distributed, or 'spread', over its surface in certain ways, for instance snow over the roof surface, vehicles over a bridge deck etc. A load distributed over a portion of the structure is termed Distributed load.
  13. 13. Two major types of load distribution are most common: UNIFORMLY DISTRIBUTED LOAD A uniformly distributed load is a load that has been spread over a beam in such a way that each unit of length has equal weight. The centre of gravity of such a load is at the point where it balances when placed on a knife edged object.
  14. 14. CONCENTRATED OR POINT LOAD A point load refers to a point where a bearing or structural weight is intense and transferred to the foundation. A good example is when a hammer applies a force at one point and regardless of what point the hammer's head hits, force is applied.
  15. 15. VALUES OF LOAD FOR DESIGN PURPOSE Load values are specified in Codes or Standards. Codes and standards are design aids as mentioned earlier.
  16. 16. SAFETY The ability of the structure and every part of it to support the load without collapsing, taking into account uncertainties in the values of actual loads and in the strength and behaviour of the structure. SERVICEABILITY The ability of the structure to ensure its satisfactory functioning. This implies particularly limitations on the magnitude of movements under various applied loads (Deflection, vibration etc.).
  17. 17. MOVEMENT Movement is the result of the action of force, or a combination of forces. In general, movement can include such parameters as distance, speed, time, acceleration. In the context of this work only the distance is of interest, both in its own right and as an indicator of the force causing it.
  18. 18. DISPLACEMENT Displacement is the distance through which a body, or a point on the body, moves as a result of the action of force. This distance is a vector. It is characterised by a magnitude -the amount of travel -and a direction.
  19. 19. ROTATION Rotation is a kind of movement (displacement) but it is more complex than the linear movement implied so far. When an object rotates there is a point in it which does not move at all and different points on it have different displacements – different magnitudes and directions of distance.
  20. 20. FORCE COUPLE AND MOMENT A rotation cannot be affected by a single force vector of the type we have encountered. Since the body as a whole does not move, there can be no net force acting on it (see force Resultant below). We can imagine a rotation of a body if the body is acted upon by two forces of equal magnitude (say P) and opposite direction, such that the lines of action of the two forces are offset by a certain distance (a, say). Such a pair of forces is termed a Force couple, or Couple for short. The body as a whole cannot move, because the two forces act in opposite directions. But at each of the two points of application of forces, the corresponding force moves the point in its direction. The result is that the two points move in opposite directions, causing the rotation of the body.
  21. 21. MOMENT Moment is defined as the perpendicular distance from a point to a line or a surface, and is derived from the mathematical concept of moments . It is frequently used in combination with other physical quantities as in moment of inertia, moment of force, moment of momentum, magnetic moment and so on. Moment is also used colloquially for different physical quantities that depend upon distance. For example, in engineering and kinesiology the term moment is often used instead of the more complete term moment of force. A moment of force being the product of the distance of a force from an axis times the magnitude of the force, i.e., F × d, where F is the magnitude of the force and d is the moment of the force. See torque for a more complete description of moments of force or couple for the related concept free moment of force also known as a force couple. It may also be used when the distance is squared, as in moment of inertia. The moment of inertia is the "second moment" of mass of a physical object. This is the object's resistance or inertia to changes in its angular motion. It is roughly the sum of the squared distances (i.e., moments) of the object's mass particles about a particular axis .
  22. 22. RESULTANT FORCE Normally a structure is not subjected to a single force, but to .a combination of several loads and other forces, in different directions and locations. In order to understand how the structure responds to such load combinations, it is necessary to know how to handle such combinations -how to operate with vectors.
  23. 23. SUMMALATION OF VECTORSRESULTANT When a number of forces (or any vectors) act on an object simultaneously, the Resultant force (or Resultant vector) is a single force (vector) which, if acting alone on the object would have the same effect as the combined forces (vectors). It is said to represent the sum of the vectors, or the Vectorial sum. It is easy to visualise a resultant vector and a way to derive it if we think of displacements rather than forces. If we think of each vector as a corresponding displacement, and instead of applying them simultaneously apply them sequentially (the final result being the same), then the resultant displacement is the distance from the starting point to the final point. To obtain the resultant graphically, plot the individual vectors tail to head. The resultant is the vector joining the tail of the first vector with the head of the last.
  24. 24. RESULTANT FORCE PARALLEL FORCE The magnitude of the resultant of a set of parallel forces is simply the sum of the forces and the direction is parallel with the forces. The question is the location of the resultant relative to a reference point. To obtain the location of the resultant force, apply at the reference point imaginary forces of equal magnitude and opposite sense to the given forces. These imaginary forces form couples with the original forces. Their sum forms a couple with the resultant force. The location of the resultant force is determined from the condition that its moment is equal to the sum of the moments of the given forces. This is because the effect of the resultant has to be the same as that of the given forces in every respect, including rotation with respect to any point.
  25. 25. GENERAL SYSTEM OF FORCES This expression can be used to obtain the location of the origin (the application point) of the resultant of any set of forces (not necessarily parallel), by working with their components. Each force is replaced by its components, having the same point of application as the force. The components parallel to any axis (x , y) form a set of parallel forces and so the expression above gives the location of the component of the resultant parallel to the same axis (i.e. its distance from the axis). The origin of the resultant is at the intersection of the directions of the two components.